Patent GB-648674-A (GB)
Title: Improvements in or relating to systems for electrical communication over a common medium
Inventor: BELLAIRS GUY FFARINGTON
IPC: H04M7/16
Filed: 1947-01-27
Abstract: 648,674. Automatic exchange systems. BRITISH TELECOMMUNICATIONS RESEARCH, Ltd. Jan. 27, 1947, Nos. 2525 and 6292. [Class 40 (iv)] [Also in Group XL (c)] In a high-frequency system similar to that described in Specification 455,279, a call is set up over a plurality of " mains " in tandem and the frequency of the carrier wave on one at least of the "mains" is controlled by the calling party. In the system described, numbers containing five digits are used. The impulses of the first two digits step switches at the local exchange to send out a carrier having a frequency characteristic of the wanted exchange. At the wanted exchange, the carrier is detected and the last three digits step switches to apply to other " mains " a carrier characteristic of the wanted party. In general, a desired frequency is selected or detected from a number of frequencies present by a heterodyne method. The wanted carrier frequency (N) along with any other frequencies present is combined with a marker frequency (T) in a modulator M. The resultant intermediate frequencies (N-T and others) are applied to either a low-pass filter LPF or an intermediate frequency filter IFA and a detector D which responds only to a standard intermediate frequency (n). Components of the system not specifically mentioned. herein are denoted by symbols M for modulator D for detector IFA and LFA for intermediate and low-frequency amplifiers LPF and BPF for low-pass and band-pass filters. In the example described, separate " go " and " return " co-axial cables are used but a single cable may be used for both directions. The " go " and " return carriers for a station differ by a standard intermediate frequency. Power may be supplied to substation and exchange apparatus over the telephone cables which, according to the earlier Provisional Specification may be used simultaneously for other services such as radio, television, facsimile, telegraphy, time signals, fire alarms, &c. Gasdischarge tubes may be used to detect calling signals in a substation and to count impulses at the exchange. The Figures, when arranged as in Fig. 11, show a complete system. Substation circuit, Fig. 1 : the " go " carrier frequency characteristic of the substation is generated by a crystal controlled oscillator CCOI and is fed to modulators MIA, MIB, the latter being effective only when the switch hook SW is up. The carrier is modulated at MIB with voice-frequency representing impulses dialled at DS or speech from a microphone MCM. The " return " carrier, when combined with the marker frequency from CCOI in MIA operates either the bell B or the loudspeaker according to the position of the switch hook SW. A gas-discharge tube may be fired by the " return " carrier to ring the bell. Alternatively, the carrier may be modulated with ringing current. The circuit is described and elaimed in Specification 648,707. Frequency-finder (line-finder), Fig. 2 : at the exchange, a variable frequency oscillator VFO2, under the control of a sweep oscillator SO and a control device CD, makes a continuous search over the whole range of frequencies used in the " mains " for a calling or " go " carrier on CCIB with no corresponding " return " carrier on CCIA. According to the earlier Provisional Specification, the oscillator VFO2 may be arranged so that the frequency changes in steps. The " return " carrier is detected by relay B which, when operated, disables relay A. The " go carrier operates relay A if B is not operated. When A operates, the sweep oscillator is cut off at A1 and a discriminator DCS and a control device CD co-operate to adjust VFO2 so that the intermediate frequency falls in the middle of the pass band of IFA2. Dialling tone is applied at A3, Fig. 3. An oscillator of 2000 c/s. at M2C modulates any frequencies on CCIA and spreads them so that A is disabled not only by a return carrier of frequency corresponding to the setting of VFO2 but by proximate carriers. The carrier frequency on CCIA is provided by VFO2. Voice frequencies pass between the modulators M2A, M2D in the frequency-finder and modulators M3E, M3C in the exchange-frequency selector (Fig. 3). According to the earlier Provisional Specification intermediate frequencies may be used at this stage in a single exchange system. The frequency finder is described in detail in Specification 648,708. Exchange-frequency selector, Fig. 3 : an impulse relay AA responds to the impulses and operates successively the magnets MTT, MT to register the first two digits on the switches TTS, TS of a frequency generator DFG3. Switch HS serves as a hunting switch. Contacts of the three switches are supplied with frequencies characteristic of tens of thousands, thousands and hundreds. The sequence switch SS (magnet MS) is stepped by relay CC which falls back during an interdigital pause. When SS reaches the third contact, the frequencies from TTS, TS and from the first contact of HS combine to provide a calling frequency representing the first of a number of carrier frequencies giving access to the wanted exchange. The second and third such frequencies are obtained when switch HS is moved to the second and third contacts. Any carrier frequency already in use is spread by modulation at 500 c/s. at M3F and is detected by D3A when combined with the marker frequency supplied to M3D over switch SS and contact T1. Relay GG operates, holds relay T which operated with CC and closes a circuit to the magnet MH to step the switch HS to find an idle frequency. When an idle frequency is found, relays GG and T fall back and the calling frequency is applied to M3E and M3C to serve as a carrier in the forward direction and to produce on intermediate frequency at M3C in the return direction. When relay A (Fig. 2) falls back at the end of a call, homing circuits are completed successively for the switches HS, TS, TTS over contact A4 (Fig. 3). According to the later Provisional Specification the switches HS, TS, TTS may be replaced by an arrangement of gas discharge tubes which are fired by impulses. The selector is claimed in Specification 648,709. Final selector, Fig. 4 : cables LL3A, LL3B and JCC3A, JCC3B give access to final selectors at the local exchange and at distant exchanges respectively. Each final selector responds to only one exchange-frequency marked by an oscillatot FO4. The calling frequency on LL3A is detectedby D4A and operates relay AB to connect the return frequency to LL3B. Impulsing. relay BA repeats the impulses at BM to register the wanted-station digits on the switches HDS, TNS, US of a frequency generator DFG4 similar to DFG3. If the wanted-station frequency is already present on CC4A, it is detected by D4B and relay G operates. Busy tone is applied to LL3B over G1. If the frequency is idle, TA falls back and the wanted-station frequency is transmitted over TAI to " mains " CC4A. Ringing current supplied over db3 modulates the carrier at M4G and the wanted station responds as described with reference to Fig. 1. When the called party replies, relay DD operates and is followed by DA, DB to send a momentary VF signal over d1, da 1 to operate DC, Fig. 3. Relay DC operates the metering device described in Specification 648,710. Fig. 5 shows a series of exchanges EX2 &c. connected through matching networks MN2 &. to a " mains provided with repeaters RP1.RP2. PBX final selector, Fig. 4 : if a number of carrier frequencies are allotted to the same number, each frequency but the last is marked by a marking frequency MO1 &c. spaced from the marked frequency by 2000 c/s. The filter BPF4C passes 2000 c/s. and relay H is operated by detector D4C whenever DF4G produces a marked frequency. If the marked frequency is busy G operates as described and G, H operated together close a circuit over G3, H2 for magnet MU to stop the switch US until an idle carrier frequency is found, indicated by the falling back of G. Relay H at H1 prevents the connection of busy tone until the last and unmarked frequency is tested and found busy. Connection to an exchange with mechanical switching, Fig. 5A : the exchange EX5 employs mechanical switching and is connected to the " mains JCC3A, B, over separate spurs SA, SB and PA, PB for incoming and outgoing calls. The equipment TE5A corresponds to the final selector of .Fig. 4 and the relay B5A controls the impulsing into the mechanical equipment MSE5A to which the called subscriber S5A is connected. The hybrid coil HC5 is provided with a balancing network BN5. For outgoing calls the mechanical equipment MSE5B is taken into use by the subscriber and the impulse repeater IR5B sets a frequency generator in OG5B to select an idle frequency allocated to the exchange required. Matching networks MN5A, B are provided. Modifications : frequency selectors may be arranged back-to-back (Fig. 7, not shown). The calling frequency is found by the first selector. A frequency generator is set by the dialled impulses and injects the set frequency into the second selector in the backward direction. The second selector finds the frequency and emits the frequency of the wantedstation in the forward direction. To avoid using a very wide band transmission in one " mains ", two or more " mains " such as JCC3A, B and J5C, D, Fig. 5, may each connect 500 subscribers spread over the same frequency range. At the exchange, the transmission on JCC5C, D may be group modulated at GMA, B and filtered so that the 1,000 channels of the two "mains" " may take up consecutive positions at the exchange. According to the earlier Provisional Specification (Fig. 2, not shown), one stage of selection may be carried out by mechanical switches such as relays.